Heat-Resistant Fungi: Importance and Current Outlook

Occurrence, Detection, and Molecular and Metabolic Characterization of Heat-Resistant Fungi

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Fungi are classiﬁed mainly by their morphologic characteristics, that is

the way observations of ascospores and anamorph morphology are important for identifying heat-resistant fungi. It needs a long time to form ascospores, however, special knowledge and techniques are required to

identify them by their morphologic characteristics. Therefore, rapid and

simple methods of identiﬁcation and discrimination of heat-resistant fungi,

like Neosartorya ﬁscheri, which is the most signiﬁcant agent, are required in

the food industry (Yaguchi et al., 2012). In the food industry, rapid detection and identiﬁcation of microorganisms or contamination sources is

required to take decisions concerning the loss of raw material and production lots. This also minimizes the costs of unnecessary storage. Conventional

detection and identiﬁcation of molds are based on cultivable isolates and

the plantation of plants such as palm, sugar cane, rice, cotton, potatoes,

barley, and banana (Malloch and Cain, 1972).

Hamigera and Thermoascus belonging to the order Eurotiales form ascospores that are highly heat resistant (Hosoya et al., 2014; Nakayama et al.,

2010). The major species of Thermoascus genus includes Thermoascus crustaceus, Thermoascus thermophiles, Thermoascus aurantiacus, and Thermoascus aegyptiacus (Houbraken and Samson, 2011). These fungi were isolated from

Occurrence, Detection, and Molecular and Metabolic Characterization of Heat-Resistant Fungi

rDNA are efﬁcient for species- or genus-level identiﬁcation of most fungi

and primers targeting the calmodulin gene in Aspergillus and Neosartorya

identiﬁcation. Calmodulin gene was also reported as a method of identifying

ochratoxin-producing fungi (Susca et al., 2007). Increasing number of the

heat-processed food and beverages spoilage by heat-resistant fungi had inﬂuence on the development molecular-based methods for such fungi detection. The study on microtubules composition of Byssochlamys fungal

strains, one from the most heat-resistant fungi, indicated that globular

proteins such as a- and b-tubulin are main components of microtubules.

Because of low level of intraspeciﬁc DNA variation in the b-tubulin gene

(total length of about 2.000 bp), this region is useful for species-level identiﬁcation of different fungal genus, including Byssochlamys (Nakayama et al.,

2010). The variation of PCR techniques is duplex or multiplex PCR techniques. The same melting temperature of primers for different species or

genus could be allowed to make PCR reactions under the same conditions

as duplex or multiplex PCR. A crucial point for functional diagnostic PCR

is the availability of unique target sequences, because if such speciﬁc DNA

sequence can be detected by PCR, it can be concluded that the fungus contaminates the sample. Genomics is much extended and whole genomes

become accessible from various fungal species. It gives a chance to propose

primer sequences, which can be checked for possible cross-hybridization by

means of genomic database (Geisen, 2007). Conventional PCR can be used

only to evaluate the presence of fungi, without information about cells or

spores number in the sample and there is no information about micotoxins